The present invention relates to a cathode ray tube camera of the type having an imaging plate comprised of optical fibers that are coated on one side with a phosphor layer.
While it is possible to photograph a color television screen or a monitor with a simple camera, the result that is obtained satisfies only very modest requirements. Reasons for this limitation include the facts that the number of lines per image height provides a low image sharpness, the shadow mask which separates the individual image elements for blue, green and red from each other is also reproduced, and the curved screen leads to geometrical distortions. For this reason, special cameras have been designed to photograph such images with a high resolving power. These cameras are equipped with relatively small picture tubes (the diagonal of the image is approximately 12-23 cm=5-9 inches). These picture tubes produce the image on a flat front plate, whereby geometrical distortions can be avoided. The image is transferred by optical means to the light sensitive material. The picture tube contains on the inner side of the front plate a phosphor layer which emits blue, green and red light, thereby producing an non-colored image. Blue, green and red filters can be successively inserted into the beam path. For example, the electron beam of the picture tube is first modulated with the blue signal and the image exposed through the blue filter, then with the green image combined with the green filter, followed in a similar manner by the red exposure. This method avoids the disadvantages of the shadow mask. The sharpness of the image may be significantly improved relative to color television, since available picture tubes are capable of a resolution of up to 4,000 lines per image height.
Very good images can be produced with a cathode ray tube, depending on cost, but there is a severe disadvantage: the modest luminous efficiency of the optical system, combined with the sequential exposure of the three basic colors, leads to long exposure times. With 1,000 lines per image height for a material of a sensitivity of 80-100 ISO, for example, approximately 30-60 seconds are required for the exposure. If picture tubes with a higher resolution, for example, 4,000 lines, are used, the exposure time is even much longer, since the image elements then have areas for example that are 16 times smaller, resulting in a lower light intensity.
In order to shorten the exposure time, phosphors with an increased red emission may be used, since the exposure time for red is the longest. For example, phosphors doped with europium may be employed. Also, the anode voltage of the picture tube can be increased. However, beginning at about 20 kV, x-rays are produced, which are not acceptable without further measures.
A further method to shorten the exposure time is the use of color photographic material with a higher sensitivity. Materials of this type are available in the form of camera films only, their resolution is lower and their graininess higher than that of materials with a lower sensitivity. These disadvantages limit their application. For this reason, materials with lower sensitivities, but capable of particularly rapid and simple photochemical processing are preferred.
For this application, CRT cameras were developed that make it possible to expose photomaterial in contact with the front plate of the picture tube. The optical imaging means are eliminated and the apparatus therefore is much smaller. In order to obtain good image sharpness an essential change is, however, required: the front plate of the picture tube consists of a plurality of very thin optical fibers, which conduct the light from the internal phosphor layer, which is located in a high vacuum, over a straight path onto the external photographic layer. The optical fibers are fused together into a compact, air-tight plate. The fibers therefore have a hexagonal cross section, a thickness of about 7 microns, and they consist of a core and a jacket. The individual fibers thus are at least 7 times thinner than the size of the relevant image structures in an image of 20.times.25 cm. The core and the jacket are made of glasses with different refractory indices. The core has a higher refractory index than the relatively thin jacket. The light is conducted by total reflection from the jacket. In order to absorb scatter light and light with a large angle of incidence (relative to the perpendicular to the surface), typically 3% of the fibers are made of black glass. The cross section of the black fibers and the jacket cross sections (approx. 9.5%) result in a light loss of about 12.5%.
CRT cameras with optical fiber front plates are made for the exposure of monochromatic photographic materials. In operation, successive lines are written at the same location on the long and narrow front plate. The photomaterial is moved in a suitable manner transversely to this line, so that the lines appear adjacent to each other to form a complete image after photographic processing. Due to a much higher luminous efficiency compared to conventional CRT cameras, images of for example 20.times.25 cm may be recorded typically in about 2-4 seconds, even on low sensitivity photomaterials.
However, faster optical CRT cameras capable of color work are not as yet available. It has already been proposed to coat the inside of the front plate with three strips of different phosphors adjacent to each other, one each for the emission of blue, green and red light. See, for example, U.S. Pat. No. 4,309,720. The electron beam writes successive lines, for example first blue, then green and finally red. This merely requires a vertical deflection of the beam in a manner such that it will jump for each color line from one phosphor strip to the other. It is further possible to use a CRT with three electron beam sources, one for each basic color, as is customary in "triple gun" color television tubes.
In this method, independently of whether a single beam or a three-beam tube is used, the signal must be synchronized with the motion of the photomaterial, that is, associated blue, green and red signals must be written in succession. Thus, for example, the green signal must be written later than the blue signal, and the red signal later than the green signal. This is readily possible at the present time by electronic means.
The use of three phosphor strips, one each for the colors blue, green and red, is, however, not without limitation. Compared with a tube using one phosphor only, manufacturing is more difficult and furthermore, many phosphors have emission spectra which are not sufficiently monochromatic. This unfavorably affects color separation and thus color reproduction. Data concerning the selection of phosphors is given in U.S. Pat. No. 4,459,512.
It is the object of the present invention to eliminate the aforementioned disadvantages of cathode ray tube cameras having several phosphor strips. It is a particular object to provide a color capable cathode ray tube camera having high light efficiency, thus making possible better color separation and better color reproduction.